The present invention relates to the field of producing a humidified gas stream with a precisely controlled moisture content in the gas stream.
Humidified gases such as nitrogen, non-cryogenically generated nitrogen, hydrogen, air, oxygen-enriched air, carbon dioxide, argon, helium, and mixtures thereof are widely employed by chemical, thermal, metallurgical, electronics, laser processing, fuel cells, and food processing industries to enhance chemical reactions, welding and spraying metallic and ceramic materials by thermal and plasma techniques, brazing and sintering metallic components, refining ferrous and nonferrous metals and metal alloys, enhancing combustion, providing the desired physical and mechanical properties to metals and metal alloys, soldering electronic components, depositing oxides of various elements by chemical vapor and physical vapor deposition techniques, controlling composition of gases used in lasers, manipulating composition of gases used in fuel cells, enhancing shelf life of perishable food items such as vegetables and fruits, and packaging food stuffs. They are also used in controlling the environment and adjusting comfort level for humans such as by producing and supplying synthetic breathable atmospheres and medicinal gases.
Numerous techniques have been developed and commercially used today to humidify gases. For example, a gas stream has been humidified by passing the gas stream over water placed in a vessel maintained at ambient temperature. The extent of moisture picked up by the gas stream using this technique depends upon the flow rate of the gas stream and surface area of the water exposed to the gas stream. This technique provides very limited pick up of water by the gas stream and is primarily used in applications where there is no need to control or regulate humidity of the gas stream and where the humidity requirements are not high.
To increase humidity or moisture pick up by the gas stream, a gas stream has been humidified by passing it over heated water placed in a vessel or flowing the gas stream through a screen with dripping water. Once again, the extent of moisture picked by the gas stream using this technique depends upon the flow rate of the gas stream, surface area of the water exposed to the gas stream, and water temperature. This technique is also primarily used in applications where there is no need to control or regulate humidity of the gas stream.
Numerous techniques have been employed and used to humidify gases with some type of humidity control. For example, a gas stream is split into two separate streams; one passing through the humidifier such as discussed above and the other by-passing the humidifier. The two streams are then combined and the humidity level of the combined stream is measured by a relative humidity measuring instrument. The humidity level of the combined stream is then controlled either by regulating the flow rate of the gas stream passing through the humidifier or by-passing the humidifier. Alternatively, gas streams are humidified simply by adding steam and regulating the humidity level by the extent of steam addition. Although these techniques do provide a form or type of humidity control and are suitable for many environmental control, food processing, and combustion related applications, they fail to provide the precise control of humidity that is required in many chemical, thermal, metallurgical, and electronics applications. Furthermore, they are not suitable for precisely humidifying gases with low humidity such as 2,000 ppm of moisture in the gas stream or about +8.degree. F. or less dew point measured at ambient temperature and pressure. The main reason for failure of these techniques to provide precise control of low humidity gases is unavailability of reliable low humidity production systems and measurement devices.
Gases have been humidified with a known amount of moisture without relying on humidity measuring devices by bubbling them through water. The moisture content of the gas stream humidified by passing through a bubbler is calculated from the operating conditions such as water temperature and total pressure of the bubbler. For example, the vapor pressure of water or moisture in the gas stream is determined from the water temperature. The vapor pressure of water and total operating pressure information is then used to calculate partial pressure of water or moisture content in the gas stream. The above calculation inherently assumes that the gas stream is saturated with moisture. If the gas stream is not saturated with moisture, then the calculated moisture content value will always be higher than the real moisture content in the gas stream. This is the main reason that bubblers are seldom used in applications requiring precise, consistent and reliable humidity levels.
Numerous changes in the design of bubblers have been made over the years to provide precise, consistent and reliable humidity level in gases. These improvements have been focused toward improving gas-liquid contact and maintaining constant water level and water temperature in the bubbler. Some of the new bubbler designs do provide a humidified gas stream with precise, consistent and reliable humidity levels, provided flow rate of the gas stream is maintained constant. Therefore, bubblers are sized and designed to provide a fixed flow rate of a humidified gas stream. They, however, fail to humidify a gas stream with precise, consistent and reliable humidity level if the flow rate of the humidified gas stream changes with time or if the moisture level requirement in the humidified gas stream changes with time.
Gases such as nitrogen, argon, helium, etc. have been humidified with the precise amount of moisture by adding a known amount of oxygen or oxygen present in air and reacting the oxygen with hydrogen over a precious metal catalyst. This technique is very versatile and can be used to provide a gas stream with precise, consistent and reliable humidity level even if there is a change in flow rate of the humidified gas stream with time or change in the moisture level requirement with time. However, it requires expensive hydrogen and a precious metal catalyst to humidify gases, and is thus, prohibitively expensive. This technique is not practical to humidify gases for applications in which residual hydrogen in the humidified gas stream is not desirable and at locations where hydrogen is not available. Furthermore, this technique is not applicable to humidify gases containing oxygen such as air, oxygen-enriched air, etc.
Based on the above discussion, it is clear that there is a need for a system to humidify gases with a precise, consistent, and reliable amount of moisture without relying on a humidity measuring device. Furthermore, there is a need for a system to provide a gas stream with precise, consistent and reliable humidity level for applications requiring different flow rates of humidified gases with time and/or different humidity level with time.